Directional antenna systems for receiving electromagnetic radiation have been practiced for many years. A variety of methods have been used to achieve varying degrees of success using terminated traveling wave antennas, phased arrays, parasitic arrays, and true-time delay arrays.
In practice, the antenna designer is often faced with a difficult tradeoff between complexity, gain, directivity, size and bandwidth. For example, for frequencies below 5 MHz, a terminated beverage antenna having a length of multiple wavelengths is known in the art to provide exemplary directivity over a wide bandwidth, but its size makes it difficult to deploy in many settings, especially when multiple antennas are required to achieve desired directional patterns. Rhombic antennas provide exceptional gain for a fixed pattern but also require significant support structure and real estate for effective operation. Curtain arrays provide moderate bandwidth and are moderate in real estate usage and require substantial investment in superstructure. Log Periodic arrays are known for their wide bandwidth and suitable directivity but also require significant investment in superstructure. Parasitic arrays are known for exceptional gain, excellent directivity, and moderate size, but require moderate superstructure and have a very small operational bandwidth.
Loop antennas are known in the art for providing a reliable bi-directional pattern for a relatively small size. It is well known that the signal from a loop antenna can be phased with a closely spaced vertical antenna element to achieve a cardiod pattern over a small bandwidth. In addition, including a properly selected and located resistor in series with a loop can provide a similar cardiod pattern. Other examples in the art include multiple loops in phased arrangement, being spaced apart in end fire relation.
Others have noted the value of utilizing a true-time-delay method of combining signals from two moderately spaced elements. For example, U.S. Pat. No. 3,396,398 issued to J. H. Dunlavy, Jr. teaches a two element true-time-delay antenna using a pair of shortened dipole elements separated by preferably less than 0.3 times the length of the shortest wavelength handled by the system. Such and antenna promises to provide exceptional bandwidth and reasonable directivity. However, the size of such an array is still considerable if, for example, if the shortest wavelength is twenty meters, the length of the dipole elements is six meters with a separation between elements of three meters.
The present invention provides a refreshing option for the antenna designer by providing a compact antenna having structural simplicity, acceptable gain, respectable directivity, fractional size, and exceptional bandwidth. For example, a single loop embodiment having a base length of seven meters provides an operational bandwidth of 0.5-14 MHz. A dual loop embodiment with each loop having individual base lengths of 3.5 meters each, and a separation distance of three centimeters provides an operational bandwidth of 1-22 MHz.
In addition, the nature of the arrangement of the loops and associated structure lends itself to configuring orthogonal arrays that can be electronically switched to provide means to rotate the pattern without physical rotation. These and other advantages the present invention will become apparent from a thorough review of this specification.